Ranked in these QuestionsQuestion Ranking
Pro CoffeeScript and SCSS support
Pro Allows collaborating on code
JSFiddle makes it easy to save the code snippets and send the link to others who can view and edit the code.
Pro Supports a wide variety of frameworks and extensions
JSFiddle can switch to selection of frameworks and extensions including jQuery, AngularJS, ReactiveJS, D3 simply from a dropdown.
Pro Great source from Google search's perspective
Pro Easy to learn
All the tutorials are written in a straightforward and easy to understand way.
Pro Built in editor
Almost every example has a "try it yourself" button which opens up an editor in a new tab. It allows you to play with the example code and see how it works.
Pro Well organized tutorials
All of the lessons are separated into their own pages, which makes it easy to learn about specific concepts.
Con Not actually an IDE
JSFiddle is not actually an IDE and is only suitable for small blocks of code.
Con Non free/libre (proprietary)
Con Outdated practices / problem solutions
The practices that are shown to solve the problems at hand are rarely, if at all, updated. Usually, their tutorials and learning material is updated only after they see their profits drop.
Con Doesn't care about teaching right
There are multiple errors in the data they show. Although the solutions they show work, they will lead to unmaintainable code. That happens even when the maintainable code alternatives are as easy or accessible to new programmers as the alternatives.
Con Certifications not recognized
Many professionals in IT agree that w3s certifications are not recognized by them and are deemed useless. Good luck finding any respectable professional that accepts a w3s certification.
Con Written tutorials only
While many learning resources offer a mixture of media in their courses (such as videos, challenges etc.), w3schools offers only written tutorials and code editors. This makes w3schools more beneficial as a quick reference rather than a primary learning resource.
Con It is for profit
What defines what goes is and what gets fixed on w3schools is what gives them profit and what doesn't (through their ads system).